In the present specification, reference is made to the following publications cited for illustrating prior art techniques, in particular conventional non-linear optics and techniques of spectrally broadening of laser light pulses.    [1] J. M. Dudley et al., “Ten years of nonlinear optics in photonic crystal fibre”, Nature Photonics 3, 85-90 (2009);    [2] N. Savage, “Supercontinuum sources”, Nature Photonics 3, 114-115 (2009);    [3] H. Imam, “Metrology: Broad as a lamp, bright as a laser”, Nature Photonics 2, 26-28 (2008);    [4] F. Reiter et al. “Generation of sub-3 fs pulses in the deep ultraviolet”, Opt. Lett. 35, 2248 (2010);    [5] F. Reiter et al. “Route to Attosecond Nonlinear Spectroscopy”, PRL 105, 243902 (2010);    [6] S. P. Stark et al. “Extreme supercontinuum generation to the deep UV”, Opt. Lett. 37, 770-772 (2012);    [7] M. Nisoli et al. “Generation of high energy 10 fs pulses by a new pulse compression technique”, Appl. Phys. Lett. 68, 2793 (1996);    [8] K. F. Mak et al. “Two techniques for temporal pulse compression in gas-filled hollow-core kagomé photonic crystal fiber”, Opt. Lett. 38, 3592-3595 (2013);    [9] K. F. Mak et al. “Tunable vacuum-UV to visible ultrafast pulse source based on gas-filled Kagome-PCF”, Opt. Express 21, 10942-10953 (2013);    [10] P. Hölzer et al. “Femtosecond Nonlinear Fiber Optics in the Ionization Regime”, Phys. Rev. Lett. 107, 203901 (2011);    [11] European application No. 13002465.6, not published on the priority date of the present specification;    [12] J. C. Travers et al. “Ultrafast nonlinear optics in gas-filled hollow-core photonic crystal fibers”, JOSA B 28, A11-A26 (2011);    [13] A. Nazarkin et al. “Generation of multiple phase-locked Stokes and anti-Stokes components in an impulsively excited Raman medium”, Phys. Rev. Lett. 83, 2560-2563 (1999);    [14] A. Abdolvand et al. “Generation of a phase-locked Raman frequency comb in gas-filled hollow-core photonic crystal fiber”, Opt. Lett. 37, 4362 (2012);    [15] A. Chugreev et al. “Manipulation of coherent Stokes light by transient stimulated Raman scattering in gas filled hollow-core PCF”, Opt. Express 17, 8822 (2009);    [16] A. Nazarkin et al. “All linear control of attosecond pulse generation”, Opt. Comm. 203, 403 (2002);    [17] C. Conti et al. “Highly noninstantaneous solitons in liquid-core photonic crystal fibers”, Phys. Rev. Lett. 105, 263902 (2011);    [18] S. Baker et al. “Femtosecond to attosecond light pulses from a molecular modulator”, Nat. Photonics 5, 664-671 (2011);    [19] R. A. Bartels et al. “Impulsive stimulated Raman scattering of molecular vibrations using nonlinear pulse shaping”, Chem. Phys. Lett. 374, 326-333 (2003);    [20] Mamyshev et al. Phys. Rev. Lett. 71, 73 (1993);    [21] F. Couny et al. Science 318, 118 (2007);    [22] S. Zaitsu et al. J. Opt. Soc. Am. B 22, 2642-2650 (2005);    [23] J. Ringling et al. “Tunable femtosecond pulses in the near vacuum ultraviolet generated by frequency conversion of amplified Ti:sapphire laser pulses”, Opt. Lett. 18, 2035-2037 (1993); and    [24] V. Petrov et al. “Frequency conversion of Ti:sapphire-based femtosecond laser systems to the 200-nm spectral region using nonlinear optical crystals”, Sel. Top. Quantum Electron. IEEE J. Of 5, 1532-1542 (1999).
The development of bright supercontinuum (SC) sources based on solid-core photonic crystal fibers has already revolutionized fields as diverse as frequency metrology, optical coherence tomography [12] and confocal microscopy [3], and is finding ever-increasing applications in biotechnology and the life sciences. A challenge to the further development of these sources is the extension of the short-wavelength edge into difficult-to-access spectral regions such as the deep UV (DUV) and vacuum UV (VUV), where material damage essentially rules out the use of solid-state materials. Currently a tunable broadband source in the UV/VUV spectral region does not exist. Broadband coherent sources in this spectral region could also be used for synthesizing single-cycle pulses in UV/DUV, thus extending sub-femtosecond science to a new spectral region.
Conventional techniques for generating radiation in the above short-wavelength range comprise e.g. high harmonic generation (HHG) or third harmonic generation (THG). HHG has disadvantages as it cannot produce a continuum covering the UV-VUV spectral region. Furthermore, one of its greatest drawbacks is its low conversion efficiency (10−6 at best). THG allows the generation of pulses with a broad spectrum in UV spectral region, which, however, have a limited bandwidth only, like e.g. 60 nm centred at 266 nm [4]. Furthermore, the spatiotemporal transformation of a single-cycle near-IR pulse in a pressurized quasi-static gas cell has been suggested for generating a UV-DUV supercontinuum in the energy range of 4 to 8 eV, i.e., 138 to 310 nm [5]. However, this techniques using the nonlinear pulse propagation in filaments has disadvantages in terms of an extreme sensitivity of the process to pump pulse parameters, resulting in a limited use with high energy (mJ) single cycle pump pulses in near-IR (about 4 fs). Another conventional approach is based on pulse broadening in tapered silica photonic crystal fibres allowing a frequency conversion down to 280 nm [6]. This technique has a restricted practical application as the lifetime of the system is rather short owing to cumulative colour centre damage to the glass when exposed to UV light over long periods of time. In the deep and vacuum UV, material loss and severe damage problems rule out the use of silica.
A further commonly employed technique for generating visible to IR supercontinua is nonlinear spectral broadening in α-pillary fibres filled with noble gases [7]. An ultrashort pump pulse, launched into the capillary, experiences self-phase modulation and strong spectral broadening. Large core (about 200 μm) capillaries must however be used to limit the propagation loss, which means that the waveguide dispersion of the empty capillary is only very weakly anomalous. When filled with gas (noble or Raman-active) at any reasonable pressure, the dispersion becomes strongly normal in the UV to near-IR spectral region. The result is a rapid broadening of the spectrum without any pulse self-compression. The absence of very short intense spectral features means that impulsive driving of the molecular motion in a Raman-active gas cannot occur, which in turn means that spectral super-broadening cannot be observed. Broadband-guiding hollow-core photonic crystal fibres, showing much stronger anomalous dispersion over a wide spectral window, provide a perfect solution to this problem.
The use of a noble gas in a hollow-core photonic crystal fiber (PCF) has enabled the generation of self-compressed pulses [8], emission of tunable deep-UV light [9], and plasma-driven frequency conversion [10, 11]. Hollow-core PCF is a unique host for gas-based nonlinear optical experiments as it offers low-loss single-mode guidance in a micron-sized hollow core along with pressure-tunable dispersion and nonlinearity. In previous work, noble gases have been used as Raman-free nonlinear media, permitting efficient soliton-based pulse compression where the interplay between Kerr nonlinearity and anomalous dispersion results in dramatic self-compression of an ultrashort pulse. Novel phenomena such as UV wavelength conversion and even plasma generation from 50 fs laser pulses of 1 μJ energy have been reported [12]. In a different context, HC-PCF filled with molecular gases offers excellent performance as an ultra-low threshold modulator and frequency shifter for nano- and picosecond laser pulses [14, 21].
Despite the success of solid-core PCFs in generating supercontinuum spectra from the mid infrared (IR) to the near ultraviolet (UV), the short wavelength edge of such SC sources is limited to about 280 nm [6] by material absorption and the properties of the silica glass used. Furthermore, even at deep blue wavelengths the glass suffers cumulative optical damage, resulting in deterioration of the supercontinuum. Currently a spectral gap in broadband SC sources exists between the near-UV and the vacuum-UV (VUV). Although soliton self-compression in kagomé-style hollow-core PCF filled with noble gases has been used to demonstrate the emission of dispersive waves at wavelengths from the visible to the VUV (at 176 nm), the generated light has bandwidths of order about 10 nm or less [9].
Mamyshev et al. have described another scheme that combines both frequency conversion and pulse compression [20]. The pulse perceives decreasing dispersion in a single-mode silica fiber due to the Raman-induced soliton self-frequency downshift, which results in an adiabatic soliton compression. However, this technique is restricted to a frequency down-conversion, and it does not allow a frequency up-conversion. As a further limitation, a very small compression factor was observed only (96 fs to 55 fs) due to a small Raman-induced frequency shift (1.57 μm to 1.62 μm) and a decreasing effective nonlinearity resulting from the frequency downshift.
Further previous works have excited Raman states in a gas filled fibre, in the anomalous dispersion regime, but with long pulses (usually ˜1 ns) ([21], [14]), which do not allow a supercontinuum generation. Ultrashort pulses have been used in [19] and [22] without being capable of obtaining supercontinuum spectra.